Plutonium Oxication State Transformations and Their Consequence on Plutonium Transport through Sediment During an 11-year Field Study

KAPLAN, DANIEL

Savannah River Site (S.C.)

关键词: Oxidation;    Adsorption;    12 Management Of Radioactive Wastes, And Non-Radioactive Wastes From Nuclear Facilities;    Sediments;    Composite Analysis;   

DOI  :  10.2172/835624
RP-ID  :  WSRC-TR-2004-00493
RP-ID  :  AC09-96SR18500
RP-ID  :  835624

美国|英语

来源: UNT Digital Library

【 摘 要 】

The Savannah River Site (SRS) contains a large inventory of plutonium (Pu), some of it in the form of Low-Level Waste (LLW). Much of this LLW has been or will be disposed at the E Area LLW Facility. As part of the permitting of LLW on-site disposal, SRS is required to periodically update the Performance Assessment calculations used in part to establish the facility specific amount of waste that can be safely disposed (or establish the Waste Acceptance Criteria). The objective of this project was to determine if a recent discovered change in plutonium chemistry (i.e., oxidation of PuO2, a common form of Pu waste, may form plutonium in the more mobile hexavalent form) was within the assumptions, parameters, and bases of the approved Performance Assessment WSRC (2000) and Composite Assessment, WSRC (1997). This project was initiated in 2001, and this is the final report describing laboratory and lysimeter (field) studies. Results from this year's work provided additional technical support f or the conceptual Pu geochemical model proposed for future risk-based calculations. When lysimeters containing Pu(III) or Pu(IV) were left exposed to the natural environment for 11 years, essentially all of the sediment-bound Pu existed as Pu(IV) and possibly Pu(III), the least mobile forms of Pu. This result was confirmed by two independent measurements, a very sensitive, indirect wet-chemistry method and a less-sensitive, direct spectroscopic technique, micro-X-ray adsorption near-edge structure (micro-XANES) spectroscopy. In these lysimeters, Pu sediment concentrations decreased on average an order-of-magnitude per centimeter for the first 5 cm below the source, an astounding rate of contaminant retardation. When Pu(VI), the more mobile form, may form from PuO2, was added to the lysimeters, the Pu moved faster than it had in the other lysimeters: Pu moving on average 12.5 cm/yr in the Pu(VI) lysimeter, compared to 0.9 cm/yr in the Pu(III) and Pu(IV) lysimeters. Importantly, transport modeling of the data clearly suggested that reduction of the original Pu(VI) occurred,thus most of the transport of the Pu in the lysimeter must have progressed during the early portion of the study, prior to the sediment-induced reduction of Pu(VI). When Pu(V) was added to the lysimeter sediment in a laboratory study, the Pu(V) quickly reduced to Pu(IV) within a couple days. These data together with those from previous reports for this project conclusively show that Pu, irrespective of the form it is introduced into SRS sediments, tends to convert rapidly to the plus 4, and possible plus 3, oxidation state, the least mobile form of Pu.